Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are widely used in manufacturing a variety of disposable and non-durable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a bond.
The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating the costly process associated with solvent removal. Such disposable or non-durable goods are usually used in mild conditions where extreme exposure to heat or cold are not normally encountered. Typical hot melt adhesives used in such applications are either based on an ethylene-vinyl acetate (EVA) copolymer or a styrenic block copolymer (SBC) such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) triblock copolymers. Hot melt adhesives consisting of amorphous poly-alpha-olefins are also used on a much smaller scale.
Hot melts also find some applications in manufacturing of OEM durable goods such as appliances and automobiles. In such applications, the structure integrity and durability at elevated as well as sub-ambient temperatures must be maintained. It is equally important that the adhesive must have long term durability to withstand environment cycles of varying temperature and humidity that are often encountered, for example, during use in an automobile. Traditionally, hot melt adhesives of choice in these applications consist primarily of polyesters, polyamides and moisture-curable polyurethanes for their high temperature resistant capability. These adhesives have provided acceptable performance in bonding engineering plastics such as polycarbonate and ABS resin to metal or to other materials over the years. These adhesives, however, are known to have a short pot life, a narrow application temperature window and high cost. In the case of moisture curable polyurethane, potential health hazards and premature curing in application equipment are also of great concern. Moreover, there is current trend to shift away from engineering plastics in favor of polyolefin type materials in the OEM durable goods industries as advanced high impact resistance polyolefins become available. Due to their poor adhesion to polyolefins, hot melts based on polyesters, polyamides and polyurethanes have become unsuitable for today's bonding needs. A need, therefore, exists for a hot melt adhesive that provides high temperature resistance, environmental durability, good adhesion to polyolefins, good application characteristics, long pot life and versatility to bind a wide range of dissimilar materials.
Various hot melt adhesive compositions containing amorphous poly-alpha-olefins (APAOs), APAO/polyethylene (PE) blends, APAO/polybutene (PB) blends, or APAO/isotactic polypropylene (IPP) blends are also known in the art. These adhesives typically consist of an APAO, or an APAO blend herein mentioned above, and a hydrocarbon type of tackifier. It is well know that adhesives based on APAOs generally have poor cohesive strength, poor heat resistance, low elevated temperature bond strength and low shear values. APAOs have not found much use for durable goods applications where a combination of high temperature resistance and high bond strength to a wide range of dissimilar substrates and easy application by conventional hot melt coating equipment is required. The APAO based adhesives usually lack such capabilities. Although various attempts were made to address these problems by blending APAO with PE, PB and the conventional IPP, very often such modifications not only failed to rectify the problems, but also led to adverse side effects of losing adhesion. These prior art APAO adhesives, consequently, usually do not have the performance properties to meet the demanding requirements of manufacturing durable goods.
For example, Trotter et al, in U.S. Pat. No. 4,022,728, describes a hot melt pressure sensitive composition comprising a mixture of APAOs, a low molecular weight substantially amorphous elastomer, a liquid tackifier and a conventional crystalline isotactic polypropylene (IPP) in the amount of up to 2% by weight. It is claimed that the composition provides good adhesive properties at low temperatures.
Meyer et al, in U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of low molecular weight polyethylene, low molecular weight conventional propylene containing polymer and APAO. These adhesive compositions are said to offer short open time and to be useful for bonding of paraffin modified corrugated board.
Lakshmanan et al, in U.S. Pat. No. 4,761,450, discloses a polymer blend useful as a hot melt adhesive comprising a low density ethylene polymer, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier and a low molecular weight polymer consisting of a low molecular weight liquid polybutene, an amorphous polypropylene and mixtures thereof.
Lakshmanan et al, in U.S. Pat. No. 5,478,891, also discloses blend compositions containing (a) a high molecular weight copolymer of ethylene with an α-olefin having at least 4 carbons and (b) an amorphous polypropylene or amorphous polyolefin. The components of the blends are described as having molecular weight ranges between 300 to 6000. The polymer blends are claimed to be useful for hot melt adhesives, coatings, sealants, asphalt modifiers and plastic additives.
Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer and a hydrocarbon tackifier.
Sustic, in U.S. Pat. No. 5,723,546, discloses a polymer blend consisting of a high molecular weight average, predominantly atactic flexible polyolefin polymer and a low molecular weight average APAO. The blend is said to be useful for hot melt adhesives.
Blending APAO with PE, PB or the conventional IPP generally leads to severe drawbacks. The prior art adhesives containing APAO/PE or APAO/PB blends, such as, for example, those described herein above in U.S. Pat. Nos. 4,120,916, 4,761,450, and 5,478,891, tend to have poor compatibility. These adhesives can undergo phase separation during their application process at which the hot melt adhesives have to be kept in the molten state at high temperature for a prolonged period of time, sometimes for hours or even days. Charring, skinning and gelling can develop rather quickly in the phase separated hot melt adhesives, thereby causing the application equipment to block or plug-up. The incompatibility of such polymer blends also imparts brittleness, optical haziness, poor or no open time, and low bond strength. Although APAO and the conventional IPP blend based hot melt do not have the compatibility problems, they still suffer from all the other drawbacks herein described above. Moreover, due to high crystallinity and high melting point of the conventional IPP polymers, hot melt adhesives based on APAO/IPP blends tend to be hard and brittle unless the IPP polymer amount is kept at a very low level, such as, for example, at about or below 2% by weight as disclosed in the prior art U.S. Pat. No. 4,022,728. As a result, these adhesives will have poor tensile strength, poor bond strength and poor impact resistance. Another detrimental effect of IPP is the increased coating temperature. The adhesive must be heated above the melting point of IPP (ranging from 180 to 200° C.) for it to reach liquid state. The blend of high and low molecular weight atactic polyolefin approach described in U.S. Pat. No. 5,723,546, although offering some improvement on tensile properties of APAO, has not been able to provide sufficient tensile strength and high temperature properties to overcome the deficiencies of sole APAO based hot melts.
A hot melt adhesive composition containing semicrystalline flexible polyolefins is disclosed by Wang in U.S. Pat. No. 6,329,468 B1 and in WO 01/53408 A1. Another hot melt adhesive composition comprising ethylene-propylene rubber and semicrystalline olefinic polymers is disclosed by Wang et al in U.S. Pat. No. 6,143,818. These compositions are well suited for disposable goods manufacturing. Due to its high amount of plasticizer requirement, however, the compositions are not capable of providing the necessary heat resistant for assembling durable goods.
In a prior U.S. Pat. No. 5,317,070, Brant et al disclosed a hot melt adhesive based on tackified syndiotactic polypropylene (SPP) having a polymer chain of at least 80% racemic dyads and having a melting point of about 100 to 180° C. The adhesive is claimed to have good open times between the application of the adhesive and the formation of the joint. This type of tackified SPP usually lacks flexibility and toughness, and therefore, will also have poor bond strength and poor impact resistance. Furthermore, SPP exhibits inherent shrinkage problems when it transforms from liquid to solid crystalline state. The shrinkage often causes stress concentration at the adhesive/substrate interface, and consequently, catastrophic bond failure.
Harandinos et al disclosed polypropylene based adhesive compositions in WO 01/46277 A2 wherein the polypropylene referred to is a class of semicrystalline copolymers of propylene with one or more other alpha olefins, and adhesive alpha-olefin interpolymers in Patent Application Publication Pub. No. US 2002/0007033 A1 wherein the polymers were largely amorphous types comprising alpha olefin monomers having C3 to C10 carbon atoms. In a related publication, WO 01/81493 A1, Faissat et al disclosed a method of processing the C3-C10 amorphous copolymer hot melts by various spray techniques. In the case of semicrystalline polypropylene based composition, the adhesive has the same shortcomings as those for the SPP compositions of U.S. Pat. No. 5,317,070. In the case of amorphous types of polyolefin, the adhesive exhibits poor heat resistance similar to all other APAO based adhesives.